The characters don't know, so there's no need to tell their players.
But why's that?
Your GM principles should give you your answer here:
- Address the characters, not the players.
- Begin and end with the fiction
- Give every monster life
Remember, your principles and agendas are rules just like HP and armor.
If you're telling the players the HP and armor of their enemies then you're breaking all three of these principles. The characters don't and shouldn't know that the Goblin over there has 3HP and 1-armor, because those aren't a part of the fiction, they're just numbers to help us interact with it. Likewise to give your monster life you need to describe it. What's more interesting?
You see a dragon on a pile of gold in the cave ahead, it has 16hp and 5-armor and it's coming towards you. What do you do?
Entering the cave the stink of smoke and roasted flesh greets you and laying comfortably atop a pile of gold is a huge red-scaled dragon, currently devouring a helpless goblin. Finishing his meal casually the dragon gets to his feet before you, that fang-laced grin that only a dragon could achieve wide across his wedge-shaped head. "Foolish Adventurers" he says in a voice that rumbles like thunder "Do you really think you are the first to try and claim my hoard? My hide has turned aside all blades brought forth against it and yours will fare no better." As he speaks you can't help but notice the myriad of shallow, harmless scrapes that mark his inch-thick scales. Smoke curls from his nostrils and his eyes gleam with menace as he takes a step towards you, leaving you feeling like a mouse staring down a cat... A huge, flying, armored, fire breathing cat that enjoys playing with his food. What do you do?
I know which one would have me reconsidering how much I wanted that treasure.
It's important to note here that the 16HP and 5-armor aren't what make a dragon a terrifying opponent, with a bit of luck a pair of level 1 characters could bring him down quite quickly if they just kept making Hack&Slash rolls until he dropped. No, the dragon is powerful because it's a goddamn dragon. A typical sword would need to find a weak spot (perhaps the belly) to even have a chance of piercing its hide at all and to make that attack the character would have to get close enough: avoiding the fiery breath, slashing claws and biting maw on the way.